1. Technical Field
Various embodiments of the present disclosure generally relate to an electronic device, and more particularly, to a semiconductor memory device.
2. Related Art
Semiconductor devices, in particular, semiconductor memory devices may be classified into volatile memory devices and nonvolatile memory devices.
The nonvolatile memory device can maintain data stored therein even when power is turned off, although read and write speeds are comparatively low. Therefore, the nonvolatile memory device is used when there is the need for storing data which must be maintained regardless of supply of power. Representative examples of the nonvolatile memory device include read-only memory (ROM), mask ROM (MROM), programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), flash memory, phase-change random access memory (PRAM), magnetic RAM (MRAM), resistive RAM (RRAM), ferroelectric RAM (FRAM), etc. Flash memory may be classified into NOR type and NAND type memory.
The flash memory has both advantages of RAM in which data is programmable and erasable and advantages of ROM in which data stored therein can be retained even when power is interrupted. Such flash memory is widely used as a storage medium of portable electronic devices such as a digital camera, a personal digital assistant (PDA) and MP3.
The flash memory may be classified into a two-dimensional semiconductor device in which strings are horizontally formed on a semiconductor substrate, and a three-dimensional semiconductor device in which strings are vertically formed on a semiconductor substrate.
Three-dimensional semiconductor devices are memory devices which are devised to overcome the limitation in the degree of integration of two-dimensional semiconductor devices and include a plurality of strings which are vertically formed on a semiconductor substrate. Each string includes a drain select transistor, which is coupled in series between a bit line and a source line, a plurality of memory cells, and a source select transistor.